Supplemental Financial Information Exploration and Production (Details 6) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Future Cash Inflows	$ 4,188,000	$ 3,790,700	$ 2,991,200
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Future Production Costs	1,560,300	1,393,000	1,095,600
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Future Development Costs	285,300	312,500	315,000
Discounted future net cash flows before income taxes	2,342,400	2,085,200	1,580,600
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Future Income Tax Expense	531,100	432,800	291,000
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Future Net Cash Flows	1,811,300	1,652,400	1,289,600
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, 10 Percent Annual Discount for Estimated Timing of Cash Flows	832,500	756,300	630,800
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Standardized Measure	$ 978,800	$ 896,100	$ 658,800